1. Field of the Invention
The present invention relates to a semiconductor device and method of fabrication, and particularly to the technology that is effective of being applied to a semiconductor device having a DRAM (Dynamic Random Access Memory).
2. Description of Related Art
A DRAM is formed by arranging memory cells on a semiconductor substrate in a matrix manner. A memory cell includes a selective transistor and an information accumulation device (hereinafter referred to as a capacitor) connected thereto. In order to form a large capacity of DRAM, it is necessary to make the capacitance of the memory cell capacitor into high density. The art for the purpose as described uses a capacitor made of a dielectric material having a large dielectric constant. A silicone is used for a lower electrode. It is known that when this silicon is heated in an oxidative atmosphere during formation of an insulator, a dioxide silicon film is grown on the surface thereof to bring forth reduction in capacitance. As a method for preventing the growth of the dioxide film, for example, Japanese Patent Laid-open No. 2001-230241 discloses a method comprising the steps of introducing a material having a reducing action when a dielectric film is produced, and bringing about a chemical reaction in a reactor to form a high dielectric gate insulating film made of a metallic dioxide film on the semiconductor substrate. According to this conventional art, it is possible to completely prevent formation of the dioxide silicon film.
The inventors have studied the aforementioned DRAM capacitor relative to a semiconductor device having a large capacity such as a 1-Gbit DRAM.
The capacitor capacity accumulated in a capacitor per bit needs an amount more than a fixed value in order to prevent a soft error and read error. Suppose that an aspect ratio of a storage node is constant, it is necessary to make the capacitor dielectric film thinner in order to meet a demand for increased capacitance resulting from scaling. In general a film thickness converted to a dioxide silicon film of relative dielectric constant −3.9 is used as an index of density of capacitor capacity. Generally, the capacitor capacity of a DRAM requires an amount more than 20 fF per bit, which corresponds to that for a converted film thickness of as extremely thin as about −3 mm in the case of a high integration memory such as a 1-Gbit DRAM. However, in such a thin thickness region, a tunneling leakage current as a direct leakage current is predominant, and a leakage current increases exponentially with the reduction in physical film thickness. When the leakage current increases, the time till information once stored is lost becomes short to impair a high-speed operation. Therefore, there has been adopted a high dielectric constant material capable of making a physical film thickness large with the converted film thickness being small, and capable of directly minimizing a tunneling leakage current.
Further, the formation of such a thin insulator involves a critical problem with an interfacial formation of a high dielectric constant-film and a lower electrode. Since phosphorus is added with high concentration to polysilicon used for a lower electrode, a natural oxide film grows far faster than a normal single-crystal substrate. Therefore, it has been found that even if the technology is used in which an oxide film is not formed on the surface during the formation of an insulator, it is oxidized by oxygen in the atmosphere during conveyance between apparatuses, and a natural film oxide has been already formed on the surface. According to the studies of the inventors, it was impossible to make a natural film oxide less than 1.5 nm because of the fast oxidization of polysilicon. Further, in the subsequent step of forming a dielectric film, where an oxidative raw material is used, the growth of film oxide further progresses. Since having a dielectric constant of as low as 3.9, even the oxide film that has a film thickness of as thin as 1.5 nm greatly deteriorates the capacitance thereof. In order to increase the capacitance it is necessary to reduce the natural oxide film thickness and increase a film thickness of a material having a large dielectric constant.